FO is a membrane separation process that relies solely on osmotic pressure, which means that water molecules spontaneously penetrate through the semipermeable membrane from the feed solution side (the high water chemistry potential) to draw solution side (the low water chemistry potential). It is an efficient route to obtain pure water for drinking from wastewater or seawater. Compared to the microfiltration membrane separation technologies, ultrafiltration, nanofiltration and reverse osmosis, there are many advantages in FO technology for treatment wastewater, including (i) high desalination capacity, (ii) low trend of membrane fouling, (iii) no additional driving force and (iv) low requirements on the feed water quality. Consequently, FO processes have been widely applied in the seawater desalination, complicated wastewater treatment, power generation, food processing, and pharmaceutical concentration.
Hydration bags developed by HTI Company are one of the FO's commercial applications, which are particularly widely used as relief devices in military and emergency fields. In hydration bags, an edible draw solution (e.g., a sugar or beverage powder) is packed in a sealed bag made of a semi-permeable FO membrane. When the bags are immersed in wastewater, the pure water diffuses into the bag due to the osmotic pressure difference and the diluted draw solution is used as drinking water. In this process, FO hydration bags provide pure water for drinking directly without external power and are widely applied in wilderness survival and military fields. But hydration bags, typically using glucose, fructose or concentrated fruit juice as the draw agents, have some defects, such as the product is not pure water, the sweet water are not appropriate for everyone (e.g. diabetics) and the draw agent is disposable, to restrict its applications.
Although the FO technology is carried out extensively, the drawbacks, such as high reverse salt flux and high regeneration cost, still restrict its application. In previous researches, many materials, such as inorganic salts, natural macromolecules, synthetic macromolecules, magnetic nanoparticles, hydrophilic carbon quantum dots and so on, are developed successively. However, these draw solutions with the above shortcomings can't fully meet the FO technology's requirements. Recently, hydrogels, as an emerging draw agent, can completely avoid the reverse diffusion, attributing to the structure of three dimensional (3D) networks in polymer chains. Nevertheless, the water flux (driving force) and water recovery rate of hydrogels are much low owing to large volumes of water entrapping into the 3D networks structure and high water retention capacity.
Aerogel, as a kind of functional material with high porosity, low density and large specific surface area, has diverse physical and chemical properties according to its composition. Gao et al have directly synthesized the ultra-flyweight aerogel through physical blending, freeze-drying and reduction reaction with two-dimensional graphene oxide and one-dimensional carbon nanotube. It was called “super sponge” because it still kept its original scale and morphology after the fatigue test of 1000 cycles and maintained a better elasticity even at −196° C. and 300° C.
In this invention, a hydrophilic and compressible SA-GO aerogel has been synthesized and used as a novel kind of draw agent in FO process with advantages of completely avoiding the reverse diffusion and the process of water production and aerogels regeneration should be rapidly and cost-effectively accomplished with low-strength mechanical compression. The SA-GO aerogels should be a potential material for hydration bags to fast and repeatable product fresh water from saline water or wastewater in military and emergency response fields.